Ethanol traditionally has been produced from grain-based feedstocks (e.g., corn, sorghum/milo, barley, wheat, soybeans, etc.), or from sugar (e.g., sugar cane, sugar beets, etc.).
In a conventional ethanol plant, corn, sugar cane, other grain, beets, or other plants are used as feedstocks and ethanol is produced from sugar (in the case of sugarcane or beets) or starch contained within the corn, or other plant feedstock. In a corn ethanol facility, corn kernels are cleaned and milled to prepare starch-containing material for processing. Corn kernels can also be fractionated to separate the starch-containing material (e.g., endosperm) from other matter (such as fiber and germ). Initial treatment of the feedstock varies by feedstock type. Generally, however, the starch and sugar contained in the plant material is extracted using a combination of mechanical and chemical means.
The starch-containing material is slurried with water and liquefied to facilitate saccharification, where the starch is converted into sugar (e.g., glucose), and fermentation, where the sugar is converted by an ethanologen (e.g., yeast) into ethanol. The fermentation product is beer, which comprises a liquid component, including ethanol, water, and soluble components, and a solids component, including unfermented particulate matter (among other things). The fermentation product is sent to a distillation system where the fermentation product is distilled and dehydrated into ethanol. The residual matter (e.g., whole stillage) comprises water, soluble components, oil, and unfermented solids (e.g., the solids component of the beer with substantially all ethanol removed, which can be dried into dried distillers grains (DDG) and sold, for example, as an animal feed product). Other co-products (e.g., syrup and oil contained in the syrup), can also be recovered from the whole stillage.
In a typical ethanol plant, a number of factors may impact the yield of ethanol produced during fermentation. These factors include the efficiency of the ethanologen, the amount of starch that is converted into sugar, pH and temperature effects, and the presence of inhibitors, to name a few. Ethanol production facilities are continually striving to increase ethanol yields in order to drive overall profitability. Particularly when the cost of the feedstock is high, increasing yields may have a substantial impact upon total plant profitability.
One method of increasing ethanol yield includes optimizing operational conditions to minimize the residual starch in the beer post fermentation. The rationale behind such a strategy is that any starch that is still present after fermentation could have been converted into sugar and subsequently converted into ethanol. These operational conditions include loading varying levels of solids, adjusting fermentation pH, temperature regimes, fermentation length, enzyme additions, ethanologen inoculation amount, etc. Another strategy for increasing the ethanol yield is the removal of compounds that inhibit the production of ethanol. Such inhibitors are traditionally identified as acetic acid, glycerol, and furfural, among many others.
What is not typically recognized, however, is that the energy utilization by the ethanologen also has a significant impact upon ethanol yield. Reducing residual starches and removing inhibitors may result in ethanol yield improvements, but these methods of yield improvement do not address the energy utilization issues of the ethanologen. For example, the ethanologen may utilize sugar generated from starch to generate Adenosine-5′-triphosphate (ATP) through glycolysis, thereby generating ethanol. The ATP generated is then utilized by the ethanologen for metabolic activities. However, it is also possible for the ethanologen to utilize sugar (e.g., glucose) for cell growth, or the generation of alternate organic byproducts. In these cases, the sugar is still consumed but is consumed without the generation of ethanol. This may also result in a reduction of residual starch, but may stifle ethanol yield.